I had something similar happen: One day my oldest T1 would only work on settings 10 and above and even those were dimmer than the same settings on my other T1. No particularly rough handling, just 20-something thousand rounds over a few years.
OP,
I apologize for my off topic reply. The internet here is very slow and all I read was "Broken Aimpoint rheostat", not specifically the part where you're asking about Micros.
I've never seen one break, but I've only ever had two. Both were never adjusted after being turned on. I'm sure one of the instructors who frequent this site or a LEO armorer who issues the Micro would be a reliable source of info for this question.
Once again, sorry for the off topic reply.